358 Pen field — Use of the Stereographic Projection for 



29° 45' (1785 nautical miles) to the Scilly Islands, at the 

 entrance to the Channel. To travel by water, a course about 

 parallel to the south shore of Long Island may first be fol- 

 lowed for about 3° (180 miles) to a point off Nantucket : then 

 a great circle course marked I, figure 25, may be followed for 

 about 8° (480 miles) to a point a, about 30 miles south of 

 Sable Island : from a, the great circle course marked II extends 

 for a distance of 8° 20' on one side of the center line of the 

 protractor and 27° 44' on the other, a total of 36° 4 7 (2,164 

 miles) to a point 5, about 30 miles southwest of the Scilly 

 Islands. The total distance traveled along the courses indi- 

 cated, from New York to h, is 2,824 sea miles, every portion of 

 which may be kept track of by means of the protractor. At 

 A, figure 25, a sliding scale is shown which is intended to be 

 used between the small circles of the protractor for estimating 

 fractions of a degree. Let it be supposed that a navigator has 

 brought his ship to the point south of Sable Island, marked a> 

 figure 25 ; he has only to turn the stereographic protractor to 

 the position shown in the figure in order to find the great 

 circle course leading from a to the point off the Scilly Islands 

 marked h. Some intermediate point or points along this course 

 being indicated on the chart, the great circle which it is 

 desired to follow could be traced in pencil on the chart, using 

 a curved ruler, which any ship's carpenter could make. The 

 speed of the vessel being known by the log, the run of every 

 few hours might be checked off, measuring the distance trav- 

 eled by means of the stereographic protractor. Thus close 

 track could be kept of the position of the vessel, which, of 

 course, would be controlled by observations whenever possible. 

 If driven from the proposed great circle course by any circum- 

 stance, it would only be necessary to determine the position of 

 the ship by observations, and, by means of the protractor, to 

 find some other great circle leading to the point of destination. 

 A feature of great circle sailing is that bearings must be 

 changed at frequent intervals, the only exceptions to this being 

 when a course is due north or south on a meridian, or east and 

 west on the equator. Since angles are preserved in the stereo- 

 graphic projection, bearings may be taken with ease and exact- 

 ness from a stereographic chart. Attention has already been 

 called to methods of measuring angles, page 256, but this 

 matter is so important to the navigator that it is considerad 

 best to add still another example, showing how an angle may 

 be measured on a chart, such as that of the North Atlantic 

 Ocean. Figure 26 represents a tracing from the writer's 

 chart, figure 24, without reduction, a being the point of 

 departure on the great circle GC leading to the English Chan- 

 nel. The chart, though not on a very large scaie, is still so 



